[libc++] remove yield from atomic::wait

libcxx/include/__atomic/atomic_sync.h

@@ -16,7 +16,6 @@
 #include <__config>
 #include <__memory/addressof.h>
 #include <__thread/poll_with_backoff.h>
-#include <__thread/support.h>
 #include <__type_traits/conjunction.h>
 #include <__type_traits/decay.h>
 #include <__type_traits/invoke.h>
@@ -108,15 +107,13 @@ struct __atomic_wait_backoff_impl {
 
   _LIBCPP_AVAILABILITY_SYNC
   _LIBCPP_HIDE_FROM_ABI bool operator()(chrono::nanoseconds __elapsed) const {
-    if (__elapsed > chrono::microseconds(64)) {
+    if (__elapsed > chrono::microseconds(4)) {
       auto __contention_address = __waitable_traits::__atomic_contention_address(__a_);
       __cxx_contention_t __monitor_val;
       if (__update_monitor_val_and_poll(__contention_address, __monitor_val))
         return true;
       std::__libcpp_atomic_wait(__contention_address, __monitor_val);
-    } else if (__elapsed > chrono::microseconds(4))
-      __libcpp_thread_yield();
-    else {
+    } else {
     } // poll
     return false;
   }

libcxx/test/benchmarks/atomic_wait_1_waiter_1_notifier.bench.cpp

@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+#include "atomic_wait_helper.h"
+
+#include <atomic>
+#include <array>
+#include <chrono>
+#include <cstdint>
+#include <numeric>
+#include <stop_token>
+#include <thread>
+
+#include "benchmark/benchmark.h"
+#include "make_test_thread.h"
+
+using namespace std::chrono_literals;
+
+template <class NotifyPolicy, class NumPrioTasks>
+void BM_1_atomic_1_waiter_1_notifier(benchmark::State& state) {
+  [[maybe_unused]] std::array<HighPrioTask, NumPrioTasks::value> tasks{};
+  std::atomic<std::uint64_t> a;
+  auto thread_func = [&](std::stop_token st) { NotifyPolicy::notify(a, st); };
+
+  std::uint64_t total_loop_test_param = state.range(0);
+
+  auto thread = support::make_test_jthread(thread_func);
+
+  for (auto _ : state) {
+    for (std::uint64_t i = 0; i < total_loop_test_param; ++i) {
+      auto old = a.load(std::memory_order_relaxed);
+      a.wait(old);
+    }
+  }
+}
+
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<KeepNotifying, NumHighPrioTasks<0>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 16, 1 << 18);
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<NotifyEveryNus<50>, NumHighPrioTasks<0>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 10, 1 << 12);
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<NotifyEveryNus<100>, NumHighPrioTasks<0>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 10, 1 << 12);
+
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<KeepNotifying, NumHighPrioTasks<4>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 16, 1 << 18);
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<NotifyEveryNus<50>, NumHighPrioTasks<4>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 10, 1 << 12);
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<NotifyEveryNus<100>, NumHighPrioTasks<4>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 10, 1 << 12);
+
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<KeepNotifying, NumHighPrioTasks<7>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 4, 1 << 6);
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<NotifyEveryNus<50>, NumHighPrioTasks<7>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 3, 1 << 5);
+BENCHMARK(BM_1_atomic_1_waiter_1_notifier<NotifyEveryNus<100>, NumHighPrioTasks<7>>)
+    ->RangeMultiplier(2)
+    ->Range(1 << 3, 1 << 5);
+
+BENCHMARK_MAIN();